Chinese Chemical Letters  2016, Vol.27 Issue (02): 256-260   PDF    
Synthesis of indol-3-yl aryl ketones through visible-light-mediated carbonylation
Hong-Tao Zhanga, Li-Jun Gua,b, Xiang-Zhong Huanga, Rui Wanga, Cheng Jinc, Gan-Peng Lia     
a Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission & Ministry of Education, Yunnan Minzu University, Kunming 650500, China;
b Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University, Kunming 650500, China;
c New United Group Company Limited, Changzhou 213166, China
Abstract: A visible-light-catalyzed synthesis of indol-3-yl aryl ketones from aryldiazonium salts, CO and indoles at room temperature was developed. This process provides a useful method for the preparation of diverse indol-3-yl aryl ketones from readily accessible reactants under base-free, acid-free and transition-metalfree conditions.
Key words: Indol-3-yl aryl ketones     Carbonylation     Visible-light     Aryldiazonium salts     Indoles    
1. Introduction

The synthesis and transformation of indoles have been and continue to be a focus of research efforts for synthetic organic chemists because the indole nucleus is found in countless biologically active molecules and medicinally relevant structures [1]. Indol-3-yl aryl ketones represent an important class of nitrogen-containing heterocycles,which are the basic constituents of numerous natural products,biologically active alkaloids,functional materials and pharmaceuticals [2]. Consequently,the development of synthetic methods for the preparation of indol-3- yl aryl ketones has received considerable attention [3]. Among the numerous methods that have been developed,the most commonly used methods are Friedel-Crafts reactions [4],Vilsmeier-Haack type reactions [5],and Grignard reactions [6]. Recently,transitionmetal- catalyzed acylation of C-H bonds has been developed as promising protocols for the construction of indol-3-yl aryl ketones [7]. However,these reactions employ acids,bases or transition metals as the catalysts. The synthesis of indol-3-yl aryl ketones under acid-free,base-free and transition-metal-free conditions has not yet been realized.

Recently visible light photoredox catalysis attracted much attention due to the rich resource of visible light,the fact that it is environment-friendly,and high efficiency [8]. After having garnered little interest for about one century since being sponsored by Ciamician,reports on visible light photoredox catalysis exploded since 2008,which exhibited powerful ability in organic synthesis and material chemistry [9]. Many visible-light photoredox catalysts such as metal or none-metal complexes have been used to solve the problem of visible light absorption efficiency [10]. In these methods,readily available,reactive aryl diazonium salts have been widely utilized as a convenient aryl radical source [11]. As a result,many unusual transformations of these reagents have been carried out by photocatalysis. Recently,Wangelin and Xiao independently reported a visible-light-mediated redox reaction that affords alkyl benzoates from arenediazonium salts,carbon monoxide,and alcohols [11b,c]. They found that in the presence of a certain amount of CO,aryl radicals can be easily and efficiently converted into acyl radicals that can be further oxidized to benzylidyneoxonium salts by the oxidized dye radical cation. We envisioned that indoles might serve as a platform to trap benzylidyneoxonium ions. To explore the application of photocatalysts and to overcome the drawbacks of the synthesis of indol-3-yl aryl ketones,herein,we disclose our preliminary results on visible light-promoted transformation of aryldiazonium salts and indoles in the synthesis of indol-3-yl aryl ketones under basefree,acid-free and transition-metal-free conditions in carbon monoxide atmosphere. The method involves a redox reaction driven by visible light and catalyzed by Eosin Y (Scheme 1).

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Scheme 1.Synthetic route to indol-3-yl aryl ketones.
2. Experimental

Reagents were obtained commercially and used as received. Solvents were purified and dried by the standard methods. The melting points were determined on an XT-4 micro melting point apparatus and uncorrected. IR spectra were recorded on an EQUINOX-= spectrometer in a KBr matrix. NMR spectra were recorded on an INOVA-400 NMR instrument at room temperature using TMS as an internal standard. Coupling constants (J) were measured in Hz. Chemical shift values (δ) are given in ppm. High Resolution mass spectrometer (HRMS) spectra were recorded on a Bruker micrOTOF-Q II analyzer. A 200-300 mesh silica gel was used for column chromatography.

General procedure: To an 8 mL vial equipped with a magnetic stir bar was charged with 1 (0.3 mmol),2 (0.4 mmol),Eosin Y (1 mol%),dry MeCN (3.0 mL). The vial was purged with N2 in the dark and transferred into an autoclave with a Quartz window bottom. The autoclave was flushed three times and slowly filled with 70 atm of CO. The reaction was irradiated with external LEDs at room temperature for 16 h. After the reaction was finished,the gas was carefully released and the vial retrieved,the reaction mixture was diluted with 5 mL of H2O,extracted with ethyl acetate (10 mL × 3). The organic portion was washed with a saturated solution of brine,dried (Na2SO4) and concentrated in vacuum,and the resulting residue was purified by silica gel column chromatography (hexane/ethyl acetate) to provide the desired products 3. The characterization data of products 3 were provided in Supporting information.

3. Results and discussion

At the outset of our investigation,phenyl diazonium tetrafluoroborate 1a,N-methylindole 2a were chosen as the model substrates to survey the reaction conditions. Gratifyingly,when a mixture of 1a (0.3 mmol),2a (0.4 mmol),Eosin Y (1 mol%) in MeCN was irradiated with 5W white LED light under a CO pressure of 70 atm at room temperature for 16 h,the desired product 3aa formed in 66% yield (Table 1,entry 1). Interestingly,the yield of 3aa was dramatically enhanced to 82% when the reaction was performed with 5W green LED light (Table 1,entry 2). A brief survey of solvents such as MeCN,DMSO,THF,EtOAc and PhCF3 led to the observation that MeCN gave the highest yield of 3aa (Table 1,entries 2-6). Subsequently,various organic dyes were further investigated and the results showed that Eosin Y was more effective in producing the desired product (Table 1,entries 2,7-9). Lower pressure of CO resulted in low conversion (Table 1,entry 10). Both visible-light photoredox catalysts and visible light are essential for the reaction to take place (Table 1,entries 11-12).

Table 1
Optimization of the reaction conditionsa.

With this preliminary result in hand,the generality of the method was explored under the optimized conditions. The scope of aryldiazonium salts 1 was initially explored in the presence of Nmethylindole 2a,Eosin Y (1 mol%) under irradiation with 5W green LED light and a CO pressure of 70 atm in MeCN at room temperature,and the results are summarized in Table 2. In general,a variety of functional groups on the phenyl ring of aryldiazonium salts were compatible in this procedure,affording the desired products in good to excellent yields. The electron-donating groups such as methoxy and methyl groups on the aryl ring allowed the aryldiazonium salts to react with 2a efficiently and gave the desired products 3ab-3ac in good yields. Aryldiazonium salts bearing electron-withdrawing halogen,CF3 and NO2 groups reacted smoothly to give the corresponding products in good yields (3ad-3af). Furthermore,the substituent at the meta position on the arene group did not affect the reaction efficiency (Table 2,entry 6). However,a substituent at the ortho position of the phenyl ring gave the corresponding product 3ah in a lower yield (Table 2,entry 7). These results indicated that the steric effects affected the efficiency of the reactions. Interestingly,a polysubstituted aryldiazonium salt gave the desired product 3ai in moderate yield (Table 1,entry 8). Notably,indol-3-yl naphthyl ketones are high affinity binding to the cannabinoid CB1 and CB2 receptors. The visible-light-induced acylation process allowed the synthesis of indol-3-yl naphthyl ketones 3aj in 54% yield directly.

Table 2
Scope of aryldiazonium salts 1a.

The substrate scope was further investigated by reacting phenyl diazonium tetrafluoroborate 1a with different indoles. Apart from 2a,1-ethyl-2-methyl-1H-indole 2b smoothly reacted with 1a to give the corresponding product 3ba in 69% yield. 1,5-Dimethyl-1Hindole 2c and 6-chloro-1-methyl-1H-indole 2d were all suitable substrates generating the corresponding products 3ca and 3da in 82% and 63% yields,respectively. However,4-methoxy-substituted N-methylindole 2e gave the desired indol-3-yl phenyl ketone 3ea in moderate yield. Treatment of substrate 2f with 1a afforded the desired product 3fa in moderate yield. In addition,1-methyl-1Hpyrrolo[2,3-b]pyridine 2g could also provide the expected product 3ga in 59% yield. It was found that reactions of free NH-indoles 2h- 2j with 1a proceeded well and gave the desired 3-acylindoles 2ha,2ia and 2ja in 51%,49% and 42% yields,respectively (Table 3,entries 7-9).

Table 3
Scope of indoles 2a.

To gain insight into the mechanism of the reaction,we conducted a series of control experiments [11b,c]. It has been reported that the reaction of aryldiazonium tetrafluoroborate with N-methylindoles can deliver azo-coupling products. We evaluated the possibility of the formation of 3-arylsulfonyl-indoles by the reactions in Tables 1-3,but no azo-coupling products or related intermediates were found under our conditions. When a radicaltrapping reagent,2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (1.0 equiv.),was added to the model reaction,no aryl ketone was obtained (Scheme 2A). These results suggest that the photoreaction proceeds via a radical pathway. Furthermore,the product yields dropped precipitously when no photocatalyst was present in the reaction and/or under dark conditions. Reactions in the dark and at increased temperature (up to 80 ℃) gave no desired products (Scheme 2B),which excludes homolytic bond cleavage of the starting material to an aryl radical under these conditions.

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Scheme 2.Control experiment.

On the basis of these preliminary results,and those of previous studies [11b,12-14],we propose a mechanism shown in Scheme 3. Initially,photoexcitation of Eosin Y by visible light generates excited [Eosin Y*]. Then the electron-deficient phenyl diazonium tetrafluoroborate 1a accepts one electron from the excited [Eosin Y*]. This single-electron transfer (SET) results in the generation of a phenyl radical (Ph•) and the oxidized dye radical cation [Eosin Y+•]. The resulting phenyl radical (Ph•) is rapidly trapped by CO to give a benzoyl radical A. Further oxidation of A by [Eosin Y+•] results in the benzylidyneoxonium B. Finally,electronic trapping of B by 2a gives the desired product 3aa.

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Scheme 3.Plausible mechanism.
4. Conclusion

In summary,we have developed a visible-light-mediated carbonylation for the synthesis of indol-3-yl aryl ketones,which is a ubiquitous component of many natural products,biomolecules and materials. Most importantly,simple and readily available Eosin Y emerges as an efficient catalyst. Metal catalysts,which are often expensive and difficult to be completely removed from products and can be a challenge in the synthesis of pharmaceutical compounds,were avoided. Mechanistic studies support the sequential operation of SET reduction,carbonylation,and back electron transfer to give aroylium cations. Further investigations of the mechanism of the reaction and its applications are ongoing in our laboratory.

Acknowledgments

We are grateful for the financial support from the Educational Bureau of Yunnan Province (No. 2010Y431) and the State Ethnic Affairs Commission (No. 12YNZ05).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.10. 012.

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